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An Origin of Cooperative Oxygen Binding of Human Adult Hemoglobin: Different Roles of the α and β Subunits in the α2β2 Tetramer.

Nagatomo S, Nagai Y, Aki Y, Sakurai H, Imai K, Mizusawa N, Ogura T, Kitagawa T, Nagai M - PLoS ONE (2015)

Bottom Line: Resonance Raman, 1H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O2-binding to rHb(αH87G), but it did partially occur with O2-binding to rHb(βH92G).The quaternary structure of rHb(αH87G) appears to be frozen in T while its tertiary structure is changeable.Thus, the absence of the Fe-His bond in the α subunit inhibits the T to R quaternary structure change upon O2-binding, but its absence in the β subunit simply enhances the O2-affinity of α subunit.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki, Japan.

ABSTRACT
Human hemoglobin (Hb), which is an α2β2 tetramer and binds four O2 molecules, changes its O2-affinity from low to high as an increase of bound O2, that is characterized by 'cooperativity'. This property is indispensable for its function of O2 transfer from a lung to tissues and is accounted for in terms of T/R quaternary structure change, assuming the presence of a strain on the Fe-histidine (His) bond in the T state caused by the formation of hydrogen bonds at the subunit interfaces. However, the difference between the α and β subunits has been neglected. To investigate the different roles of the Fe-His(F8) bonds in the α and β subunits, we investigated cavity mutant Hbs in which the Fe-His(F8) in either α or β subunits was replaced by Fe-imidazole and F8-glycine. Thus, in cavity mutant Hbs, the movement of Fe upon O2-binding is detached from the movement of the F-helix, which is supposed to play a role of communication. Recombinant Hb (rHb)(αH87G), in which only the Fe-His in the α subunits is replaced by Fe-imidazole, showed a biphasic O2-binding with no cooperativity, indicating the coexistence of two independent hemes with different O2-affinities. In contrast, rHb(βH92G), in which only the Fe-His in the β subunits is replaced by Fe-imidazole, gave a simple high-affinity O2-binding curve with no cooperativity. Resonance Raman, 1H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O2-binding to rHb(αH87G), but it did partially occur with O2-binding to rHb(βH92G). The quaternary structure of rHb(αH87G) appears to be frozen in T while its tertiary structure is changeable. Thus, the absence of the Fe-His bond in the α subunit inhibits the T to R quaternary structure change upon O2-binding, but its absence in the β subunit simply enhances the O2-affinity of α subunit.

No MeSH data available.


Related in: MedlinePlus

Intersubunit interactions of deoxy Hb A.Intersubunit interactions at the interface between the α1 and β2 subunits and C-terminal region of deoxy Hb A were revealed by X-ray crystallography [46]. Tyrα42 is hydrogen bonded with Aspβ99, and Trpβ37 is hydrogen bonded with Aspα94 of G-helix in the inter-subunit surfaces, respectively. Tyrα140 in H-helix is interacting with a residue of another subunit and is also hydrogen bonded with Valα93 as the intra-subunit interaction. Tyrβ145 in H-helix is hydrogen bonded with Valβ98 as an intra-subunit interaction. The F-helix is connected to the proximal His, while the E-helix is associated with an external ligand on the distal side. Tyrα140 and Tyrβ145 are contained in the C-terminal region.
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pone.0135080.g011: Intersubunit interactions of deoxy Hb A.Intersubunit interactions at the interface between the α1 and β2 subunits and C-terminal region of deoxy Hb A were revealed by X-ray crystallography [46]. Tyrα42 is hydrogen bonded with Aspβ99, and Trpβ37 is hydrogen bonded with Aspα94 of G-helix in the inter-subunit surfaces, respectively. Tyrα140 in H-helix is interacting with a residue of another subunit and is also hydrogen bonded with Valα93 as the intra-subunit interaction. Tyrβ145 in H-helix is hydrogen bonded with Valβ98 as an intra-subunit interaction. The F-helix is connected to the proximal His, while the E-helix is associated with an external ligand on the distal side. Tyrα140 and Tyrβ145 are contained in the C-terminal region.

Mentions: The near-UV CD spectra of Hb A provide interesting information on the tertiary and quaternary structure changes that occur upon O2 binding. The CD bands of deoxyHb A consist of two components, one at 287 nm which mainly reflects the quaternary structure transition and another around 294 nm that mainly reflects tertiary structure changes. The quaternary structure transition of Hb A upon O2 binding involves structural changes in the aromatic residues in the α1-β2 contact (Tyrα42 and Trpβ37) and C-terminal regions (Tyrα140 and Tyrβ145) (Fig 11) [4–6,46]. The former changes were detected by 1H NMR and the latter by near-UV CD, while all can be detected by UVRR. It was clarified that 70% of CD band intensity around 287 nm of deoxyHb A reflects the C-terminal region (Tyrα140 and Tyrβ145) [76–78].


An Origin of Cooperative Oxygen Binding of Human Adult Hemoglobin: Different Roles of the α and β Subunits in the α2β2 Tetramer.

Nagatomo S, Nagai Y, Aki Y, Sakurai H, Imai K, Mizusawa N, Ogura T, Kitagawa T, Nagai M - PLoS ONE (2015)

Intersubunit interactions of deoxy Hb A.Intersubunit interactions at the interface between the α1 and β2 subunits and C-terminal region of deoxy Hb A were revealed by X-ray crystallography [46]. Tyrα42 is hydrogen bonded with Aspβ99, and Trpβ37 is hydrogen bonded with Aspα94 of G-helix in the inter-subunit surfaces, respectively. Tyrα140 in H-helix is interacting with a residue of another subunit and is also hydrogen bonded with Valα93 as the intra-subunit interaction. Tyrβ145 in H-helix is hydrogen bonded with Valβ98 as an intra-subunit interaction. The F-helix is connected to the proximal His, while the E-helix is associated with an external ligand on the distal side. Tyrα140 and Tyrβ145 are contained in the C-terminal region.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4526547&req=5

pone.0135080.g011: Intersubunit interactions of deoxy Hb A.Intersubunit interactions at the interface between the α1 and β2 subunits and C-terminal region of deoxy Hb A were revealed by X-ray crystallography [46]. Tyrα42 is hydrogen bonded with Aspβ99, and Trpβ37 is hydrogen bonded with Aspα94 of G-helix in the inter-subunit surfaces, respectively. Tyrα140 in H-helix is interacting with a residue of another subunit and is also hydrogen bonded with Valα93 as the intra-subunit interaction. Tyrβ145 in H-helix is hydrogen bonded with Valβ98 as an intra-subunit interaction. The F-helix is connected to the proximal His, while the E-helix is associated with an external ligand on the distal side. Tyrα140 and Tyrβ145 are contained in the C-terminal region.
Mentions: The near-UV CD spectra of Hb A provide interesting information on the tertiary and quaternary structure changes that occur upon O2 binding. The CD bands of deoxyHb A consist of two components, one at 287 nm which mainly reflects the quaternary structure transition and another around 294 nm that mainly reflects tertiary structure changes. The quaternary structure transition of Hb A upon O2 binding involves structural changes in the aromatic residues in the α1-β2 contact (Tyrα42 and Trpβ37) and C-terminal regions (Tyrα140 and Tyrβ145) (Fig 11) [4–6,46]. The former changes were detected by 1H NMR and the latter by near-UV CD, while all can be detected by UVRR. It was clarified that 70% of CD band intensity around 287 nm of deoxyHb A reflects the C-terminal region (Tyrα140 and Tyrβ145) [76–78].

Bottom Line: Resonance Raman, 1H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O2-binding to rHb(αH87G), but it did partially occur with O2-binding to rHb(βH92G).The quaternary structure of rHb(αH87G) appears to be frozen in T while its tertiary structure is changeable.Thus, the absence of the Fe-His bond in the α subunit inhibits the T to R quaternary structure change upon O2-binding, but its absence in the β subunit simply enhances the O2-affinity of α subunit.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki, Japan.

ABSTRACT
Human hemoglobin (Hb), which is an α2β2 tetramer and binds four O2 molecules, changes its O2-affinity from low to high as an increase of bound O2, that is characterized by 'cooperativity'. This property is indispensable for its function of O2 transfer from a lung to tissues and is accounted for in terms of T/R quaternary structure change, assuming the presence of a strain on the Fe-histidine (His) bond in the T state caused by the formation of hydrogen bonds at the subunit interfaces. However, the difference between the α and β subunits has been neglected. To investigate the different roles of the Fe-His(F8) bonds in the α and β subunits, we investigated cavity mutant Hbs in which the Fe-His(F8) in either α or β subunits was replaced by Fe-imidazole and F8-glycine. Thus, in cavity mutant Hbs, the movement of Fe upon O2-binding is detached from the movement of the F-helix, which is supposed to play a role of communication. Recombinant Hb (rHb)(αH87G), in which only the Fe-His in the α subunits is replaced by Fe-imidazole, showed a biphasic O2-binding with no cooperativity, indicating the coexistence of two independent hemes with different O2-affinities. In contrast, rHb(βH92G), in which only the Fe-His in the β subunits is replaced by Fe-imidazole, gave a simple high-affinity O2-binding curve with no cooperativity. Resonance Raman, 1H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O2-binding to rHb(αH87G), but it did partially occur with O2-binding to rHb(βH92G). The quaternary structure of rHb(αH87G) appears to be frozen in T while its tertiary structure is changeable. Thus, the absence of the Fe-His bond in the α subunit inhibits the T to R quaternary structure change upon O2-binding, but its absence in the β subunit simply enhances the O2-affinity of α subunit.

No MeSH data available.


Related in: MedlinePlus